Reaction of Salicyl N-Tosylimines with 2-Cyclohexenone: A Facile Access to Tetrahydroxanthenones

Abstract

The reactions of salicyl N-tosylimines with 2-cyclo­hexenone can be carried out under mild reaction conditions in the presence of PPhMe₂ (25 mol%), which provided a facile access to tetrahydroxanthenones within a few hours in most cases. In the ­reactions of salicyl N-tosylimines with 2-cyclopentenone, either the corresponding aza-Baylis-Hillman products or the corresponding cyclized products, or the mixtures of the two products, were ­obtained in moderate to good yields under the similar conditions ­depending on the salicyl N-tosylimines. A plausible reaction mechanism is discussed.

Key words

salicyl N-tosylimine - aza-Baylis-Hillman reaction - 2-cyclohexenone - 2-cyclopentenone - tetrahydroxanthenone

References

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The crystal data of 3a has been deposited in CCDC with number 266107. Empirical formula: C₂₀H₂₁NO₄S; formula weight: 371.44; temperature: 293 (2) K; crystal dimensions: 0.315 × 0.235 × 0.086 mm; crystal system: monoclinic; space group: P2 (1)/c; unit cell dimensions: a = 13.8239 (13) Å, b = 10.0788 (10) Å, c = 13.4517 (14) Å, α = 90°, β = 103.205 (2)°, γ = 90°, V = 1824.6 (3) ų; Z = 4; D _calcd = 1.352 mg/m³; F ₀₀₀ = 784; final R induces [I > 2σ(I)]: R ₁ = 0.0628, R ₂ = 0.1352.

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General Procedure.
To a flame-dried Schlenk tube was in turn added molecular sieves 4 Å (100 mg), salicyl N-tosylimine (1a, 138 mg, 0.50 mmol), THF (2.0 mL), 2-cyclohexen-1-one 2a (75 L, 0.75 mmol) and PPhMe₂ (18 L, 0.13 mmol) at r.t., and the reaction mixture was further stirred at r.t. The reaction solution was monitored by TLC. When 1a disappeared, DBU (19 µL, 0.13 mmol) was added and the reaction mixture was further stirred at r.t. The progress of the reaction was monitored by TLC. When the cyclized product 3a disappeared, the solvent was removed under reduced pressure and the residue was purified by flash chromatography (SiO₂, EtOAc-PE = 1:10) to yield 4a (86 mg, 86%) as a yellow solid.